Method of monitoring refrigerant level

ABSTRACT

The invention relates to a method of refrigerant level monitoring in a refrigerant circuit of an air-conditioning or heat-pump system having a compressor and a refrigerant which may, depending on the operating point, be operated in the supercritical range. The method includes standstill level monitoring with the compressor switched off and/or in-operation level monitoring with the compressor switched on. In the case of in-operation level monitoring, the refrigerant overheat (dTü) at the evaporator is registered and, in the event of excessive overheat, it is concluded that there is underfilling. At a standstill, the pressure and temperature of the refrigerant are registered, and it is concluded that there is an improper refrigerant filling level if the pressure (p KM ) lies below a minimum pressure value (p min ) or the temperature (T KM ) lies above a maximum saturation temperature value (T S ) with the pressure being outside a predefinable intended pressure range ([p u ,p o ]).

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application claims the right of priority under 35 U.S.C.§119(a) based on German Patent Application No. 100 61 545.7, filed Dec.11, 2000, the entire content of which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a method of monitoring refrigerantlevel (filling amount of refrigerant) in a refrigerant circuit of anair-conditioning or heat-pump system with a compressor and a refrigerantoperated in the supercritical range as a function of the operatingpoint.

In this case, a “refrigerant which may, depending on the operatingpoint, be operated in the supercritical range” is to be understood asone which, at least for some of the possible system operating states, isin the supercritical range and is therefore present in vapor form evenin the section of the refrigerant circuit from the “condenser”,(functioning at such time as a gas cooler) to the evaporator. Inparticular, the method is suitable for monitoring the CO₂ level in CO₂air-conditioning systems, which are increasingly being used in motorvehicles. The method includes standstill level monitoring, that is tosay level monitoring with the compressor switched off, and/orin-operation level monitoring, that is to say level monitoring with thecompressor switched on.

U.S. Pat. No. 4,745,765 includes in-operation level monitoring in whichconclusions are drawn about possible underfilling, by using the measuredtemperature of superheated refrigerant at the evaporator output and theambient temperature.

U.S. Pat. No. 5,481,884 describes a method which includes bothstandstill and in-operation level monitoring. To this end, when theair-conditioning or heat-pump system considered there is at astandstill, the refrigerant pressure on the suction side of thecompressor and the ambient temperature are measured. The saturationpressure associated with the measured ambient temperature is determinedand used as a reference pressure, with which the measured refrigerantpressure is compared. If the measured pressure compared with thereference pressure is too low, it is concluded that there isunderfilling. During operation of the system, temperature and pressureof the refrigerant are measured on the suction side of the compressor.The saturation temperature associated with the measured pressure isdetermined and used as a reference temperature, with which the measuredrefrigerant temperature is compared. If the measured temperature liestoo far above the reference temperature, it is concluded that there isunderfilling.

SUMMARY OF THE INVENTION

The present invention has as its principal object to provide a novelmethod of refrigerant level monitoring of the general type mentionedabove. With relatively little effort, the method permits reliabledetection of erroneous filling, i.e., both underfilling and/oroverfilling, on a refrigerant which may, depending on the operatingpoint, be operated in the supercritical range in a refrigerant circuitof an air-conditioning or heat-pump system.

In accomplishing the objects of the invention, there has been providedaccording to one aspect a method of refrigerant level monitoring in arefrigerant circuit of an air-conditioning or heat-pump system having acompressor and a refrigerant which may, depending on of the operatingpoint, be operated in the supercritical range, the method comprising: atleast standstill level monitoring with the compressor switched off,comprising measuring both the pressure (p_(KM)) and the temperature(T_(KM)) of the refrigerant, and determining whether the measuredrefrigerant pressure lies below a temperature-independent predeterminedminimum pressure value (p_(min)) or whether the measured refrigeranttemperature lies above a predetermined maximum saturation temperaturevalue (T_(S)) and the measured refrigerant pressure lies outside apredetermined target pressure range (p_(u), p_(o)).

In a preferred embodiment, the method further comprises in-operationlevel monitoring with the compressor switched on, which is carried outby measuring the refrigerant superheat (dTü) at the evaporator of thesystem, and determining whether the measured superheat (dTü) lies abovea predetermined limiting value (dTü_(G)).

According to another preferred embodiment of the invention, there hasbeen provided a method of refrigerant level monitoring in a refrigerantcircuit of an air-conditioning or heat-pump system having a compressorand a refrigerant which may, depending on the operating point, beoperated in the supercritical range, the method comprising: at leastin-operation level monitoring with the compressor switched on,comprising measuring the refrigerant superheat (dTü) at the evaporatorof the system, and determining whether the measured superheat (dTü) liesabove a predetermined limiting value (dTü_(G)).

According to another aspect of the invention, there has been provided anapparatus for refrigerant level monitoring in a refrigerant circuit ofan air-conditioning or heat-pump system having a compressor and arefrigerant which may, depending on the operating point, be operated inthe supercritical range, the apparatus comprising: at least a system forstandstill level monitoring with the compressor switched off, comprisingdetectors for measuring both the pressure (p_(KM)) and the temperature(T_(KM)) of the refrigerant, and a calculation circuit for determiningwhether the measured refrigerant pressure lies below atemperature-independent predetermined minimum pressure value (p_(min))or whether the measured refrigerant temperature lies above apredetermined maximum saturation temperature value (T_(S)) and themeasured refrigerant pressure lies outside a predetermined targetpressure range (p_(u), p_(o)).

According to another preferred embodiment, the apparatus comprises,either as an alternative or in addition to the system for standstilllevel monitoring, a system for in-operation level monitoring with thecompressor switched on, comprising detectors for measuring therefrigerant superheat (dTü) at the evaporator of the system, and acalculation circuit for determining whether the measured superheat (dTü)lies above a predetermined limiting value (dTü_(G)).

According to yet another aspect of the invention, there has beenprovided a motor vehicle embodying the apparatus described above in anair-conditioning system that employs CO₂ as refrigerant.

Further objects, features and advantages of the present invention willbecome apparent from the detailed description of preferred embodimentsthat follows, when considered together with the accompanying figures ofdrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

One advantageous embodiment of the invention is illustrated in thedrawings and will be described below. In the drawings:

FIG. 1 is a schematic block diagram showing a CO₂ air-conditioningsystem with means for CO₂ level monitoring;

FIG. 2 is a flow diagram relating to monitoring the CO₂ level for theair-conditioning system of FIG. 1, at a standstill and in operation; and

FIG. 3 is a pressure-enthalpy characteristic map for CO₂ to illustratethe refrigerant level monitoring method according to FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of the invention specifically includes level monitoring whenthe system is at a standstill, with the pressure and temperature of therefrigerant being registered. It can be concluded that there iserroneous filling, firstly, if the registered refrigerant pressure liesbelow a minimum pressure value, which can be predefined suitably. Inthis case, there is underfilling, that is to say too low a level. It canalso be concluded that there is erroneous filling if the measuredrefrigerant temperature lies above a predefinable maximum saturationtemperature value and the measured refrigerant pressure lies outside apredefinable intended pressure range. Depending on whether the detectedpressure falls below or exceeds the intended pressure range, it isconcluded that there is underfilling or overfilling, i.e., too low ortoo high a level. In this case, use is made of the fact that, in thecase of a refrigerant temperature lying above the maximum saturationtemperature, at constant temperature the pressure decreases as the levelfalls and increases as the level rises.

In a further preferred embodiment of this standstill level monitoring,the intended pressure range is defined by using a temperature-dependentpredefinable upper and/or a temperature-dependent predefinable lowerpressure limiting value, which takes account of the fact that thepressure rises as the temperature rises, given a constant level.

The method according to another preferred embodiment specificallyincludes level monitoring during operation of the system, specificallyin such a way that the refrigerant superheat at the evaporator isdetermined, i.e., the temperature rise of the refrigerant across theevaporator. It can be concluded that there is underfilling if thesuperheat exceeds a predefinable maximum value.

In a further preferred embodiment of this in-operation level monitoring,the refrigerant superheat is determined directly by measuring andforming the difference between the refrigerant temperature at theevaporator inlet and at the evaporator outlet, or indirectly with theaid of the refrigerant temperature on the evaporator inlet side and thetemperature on the evaporator outlet side of a medium that is cooled bythe evaporator, such as an air stream. Use of the latter can providesimplification in measurement terms as compared with a directmeasurement of the refrigerant temperature on the evaporator outletside.

Turning now to the drawings, FIG. 1 shows in schematic form thecomponents of interest here in an air-conditioning system, whichoperates with CO₂ as the refrigerant and, for example, can be used in amotor vehicle. The system comprises a refrigerant circuit having acompressor 1, downstream of which a gas cooler 2 is connected on thehigh-pressure side. Connected downstream of the gas cooler is anexpansion element 3, which is followed by an evaporator 4. Via acollector 5, the refrigerant, e.g., the CO₂, passes to the compressor 1again. An air stream 7 to be cooled is led over the evaporator 4 and,for example, is blown into the interior of a motor vehicle. The gascooler 2 is cooled by an air stream 15 led over it.

An air-conditioning system control unit 6 controls the operation of theair-conditioning system in a manner which is conventional and thereforenot specifically discussed. Together with associated sensor means, thecontrol unit also provides CO₂ level monitoring, which will be discussedin more detail below.

The sensor means provided for this purpose comprise a first refrigeranttemperature sensor 10 on the inlet side of the evaporator 4 and an airtemperature sensor 11 on the air outlet side of the evaporator 4.Optionally, a second refrigerant temperature sensor arranged on theoutlet side of the gas cooler 2 and/or a refrigerant pressure sensor 9,arranged between the compressor 1 and the gas cooler 2, for example, canbe provided, as indicated with dashed lines in FIG. 1.

As an alternative to the air temperature sensor 11, a third refrigeranttemperature sensor 12 can be provided on the outlet side of theevaporator 4, as indicated with dashed lines in FIG. 1. Furthermore, asan alternative to the second refrigerant temperature sensor 8, an airtemperature sensor 13 can be provided on the air inlet side of the gascooler 2, or an ambient air temperature sensor 14, as indicated withdashed lines in each case in FIG. 1. Since such air temperature sensors13, 14 are frequently present in any case for other purposes, this maymake the refrigerant temperature sensor 8 on the gas cooler outlet sideunnecessary, if the accuracy of the refrigerant temperature estimatewhich can be achieved therewith is adequate.

The air-conditioning system control unit 6 receives the associatedmeasured signals from the aforementioned sensors and evaluates thesesignals suitably for the purpose of CO₂ level monitoring at a standstilland in system operation. FIG. 2 illustrates the associated levelmonitoring method schematically as a flow diagram.

As can be seen from FIG. 2, the control unit 6 initially determines, inaccordance with the method, whether the system is operating or at astandstill, i.e., whether the compressor 1 is switched on or off (step20). If the system is operating, the control unit 6 registers therefrigerant temperature T_(KVE) on the evaporator inlet side via theassociated temperature sensors 10, 11, and the temperature T_(LVA) ofthe air stream 7 led over the evaporator 4 on the air outlet side of theevaporator 4 (step 21). From this, the control unit 6 determines thesuperheat dTü at the evaporator 4 by using the relationship

dTü=T _(KVA) −T _(KVE) =F·(T _(LVA) =T _(KVE)),

where T_(KVA) designates the refrigerant temperature on the evaporatoroutlet side and F a proportionality factor (step 22). If the optionalrefrigerant temperature sensor 12 is present there, then the latter canbe used to measure the refrigerant temperature T_(KVA) on the evaporatoroutlet side directly, and in order to determine the superheat dTü at theevaporator 4 its definition equation dTü=T_(KVA)−T_(KVE) can then beused directly. Alternatively, use can be made of the fact that thesuperheat dTü at the evaporator 4 is proportional to the differencebetween the air outlet temperature T_(LVA) and the refrigerant inlettemperature T_(KVE) at the evaporator 4, the proportionality factor Fdepending on the quantity of air led over the evaporator 4 and thereforeon the set output of an associated air blower (not shown), since thequantity of air influences the air-side transfer of heat at theevaporator 4.

After the superheat dTü at the evaporator 4 has been determined in oneor the other way, the control unit 6 determines whether theinstantaneous superheat dTü determined lies above a predefined limitingvalue dTü_(G) of, for example, 5 K (step 23). If this is the case, it isconcluded that there is underfilling, i.e., too low a refrigerant levelin the refrigerant circuit, and the control unit 8 outputs appropriateunderfilling information (step 24).

If the system is at a standstill, the control unit 6 registers therefrigerant temperature T_(KM), for example, via the refrigeranttemperature sensor 10 or 12 on the evaporator, and the refrigerantpressure p_(KM) in the refrigerant circuit, directly via the pressuresensor 9 or indirectly, for example by registering a temperature (step25). Alternatively, in order to register the coolant temperaturedirectly, depending on the sensor equipment of the system, an indirectdetermination of the same can be provided by using the temperature ofthe ambient air, measured via the ambient air temperature sensor 14,and/or the temperature of the air stream 15 led over the gas cooler 2,measured by the associated air temperature sensor 13, at the air inletside of the gas cooler 2. The air temperature measured by the sensor 14or by the sensor 15 may permit an adequately accurate estimate of therefrigerant temperature when the system is at a standstill, inparticular following a relatively long system standstill or taking intoaccount the time period which has elapsed since the system was switchedoff.

The control unit 6 then determines whether the registered refrigerantpressure p_(KM) is greater than a predefinable minimum pressure p_(min)of, for example, 15 bar (step 26). If this is not the case, this resultis again judged to mean underfilling of the refrigerant circuit, and thecontrol unit 6 outputs the underfilling information (step 24).

If, on the other hand, the measured refrigerant pressure p_(KM) liesabove the minimum pressure p_(min), the control unit 6 next determineswhether the registered refrigerant temperature T_(KM) lies above apredefined maximum saturation temperature T_(S) (step 27). The latter isdetermined by a predefined intended density or optimum density of therefrigerant in the refrigerant circuit, which is around 250 kg/m³, forexample, and results from the internal volume of the refrigerantcircuit, for example, about 2 dm³, and the desired intended quantity ofCO₂ of about 500 g, for example. At the same time, therefore, themaximum saturation pressure p_(s) associated with the predefinableintended density is therefore defined.

FIG. 3 illustrates these conditions by using the pressure-enthalpycharacteristic map for CO₂. For the purpose of orientation, the limitlines for saturated vapor L1 and saturated liquid L2 and thecharacteristic curve of the critical temperature T_(krit) arereproduced. The latter represents the limiting temperature above whichthe CO₂ can be present only in gaseous form. Also shown is the densitycharacteristic curve for an assumed optimum density D_(opt) of 250kg/m³, whose point of intersection A with the limit line L1 forsaturated vapor represents the maximum saturation pressure p_(s)belonging to this intended density D_(opt) and the associated maximumsaturation temperature T_(S).

If the registered refrigerant temperature T_(KM) is not higher than themaximum saturation temperature T_(S) belonging to the predefinedintended density D_(opt), the CO₂ is within the two-phase coexistenceregion. Since, in this region, as the level rises and falls, the vaporcontent increases and decreases and, in the process, the pressure andtemperature of the refrigerant remain constant, it is not possible todraw any conclusions about the level from the measured pressure andtemperature data obtained, and thus there will be no statement in thisregard. If, on the other hand, the registered refrigerant temperatureT_(KM) lies above the maximum saturation temperature T_(S), that is tosay the CO₂ is outside the two-phase coexistence region, then atconstant temperature, the pressure falls as the level falls and,consequently, as the density D falls, as can be seen from thecharacteristic map of FIG. 3.

This property is used for level monitoring in this specific systemstate. The control unit 6 determines whether the measured CO₂ pressurep_(KM) is less than a lower limiting value p_(u), which corresponds to apredefined minimum density D_(min) and consequently to a specificminimum level. Therefore, the determination depends on the currentrefrigerant temperature, as illustrated in FIG. 3 by the associateddashed characteristic curve for p_(u) and D_(min) (step 28). If themeasured refrigerant pressure p_(KM) falls below the predefined lowerpressure limiting value p_(u), the control unit 6 recognizes this asunderfilling and generates the corresponding underfilling information(step 24).

If the measured refrigerant pressure p_(KM) lies above the lowerpressure limiting value p_(u)(T_(KM)) belonging to the currentrefrigerant temperature T_(KM), the control unit 6 also determineswhether it lies above a predefinable upper pressure limiting valuep_(o), which corresponds to a predefinable maximum density D_(max). Seethe associated dashed characteristic curve in FIG. 3. Consequently, thedetermination is again dependent on the current refrigerant temperatureT_(KM) (step 29). If p_(km)>p_(o), it is concluded that there isoverfilling, that is to say too high a level, so that the control unit 6generates corresponding overfilling information (step 30). On the otherhand, if this is not so, then the refrigerant pressure p_(KM) lies inthe tolerable range between the lower pressure limiting value p_(u),corresponding to a predefinable minimum density D_(min) of 150 kg/m³,for example, and the upper pressure limiting value p_(o), correspondingto a maximum density D_(max) of 350 kg/m³, for example.

As becomes clear from the above description of a preferred embodimentthe method according to the invention provides reliable level monitoringfor a refrigerant, such as CO₂, circulating in a refrigerant circuit ofan air-conditioning or heat-pump system (operated in the supercriticalrange) as a function of the operating point, both when the system is ata standstill and when the system is operating. If underfilling oroverfilling is detected, appropriate warning information is generated.The latter may be used for various further actions. For example, thewarning information can be displayed, for example, optically, via acontrol lamp. In addition, if a degree of underfilling is detected whichis critical for operational safety during operation, automatic systemdeactivation can be carried out, for example, by switching off thecompressor by disengaging an associated clutch or, in the case of aclutchless compressor, by switching to short-circuit operation.

The method according to the invention is also suitable in particular forCO₂ air-conditioning systems having a low-pressure collector with an“orifice tube”, as it is known, and/or with high-pressure control, suchas are used in motor vehicles. Depending on the application,modifications of the method variant shown are possible and areconsidered to fall within the scope of the invention defined by theappended claims. For example, in further method variants, only thestandstill level monitoring or only the in-operation level monitoringcan be implemented.

What is claimed is:
 1. A method of refrigerant level monitoring in arefrigerant circuit of an air-conditioning or heat-pump system having acompressor and a refrigerant which may, depending on the operatingpoint, be operated in the supercritical range, the method comprising: atleast standstill level monitoring with the compressor switched off,comprising measuring both the pressure (p_(KM)) and the temperature(T_(KM)) of the refrigerant, and determining whether the measuredrefrigerant pressure lies below a temperature-independent predeterminedminimum pressure value (p_(min)) or whether the measured refrigeranttemperature lies above a predetermined maximum saturation temperaturevalue (T_(S)) and the measured refrigerant pressure lies outside apredetermined target pressure range (p_(u), p_(o)), in each case as anindication of improper filling.
 2. A method as claimed in claim 1,wherein the target pressure range is limited downward by atemperature-dependent predetermined lower pressure limiting value(p_(u)) and/or upward by a temperature-dependent predetermined upperpressure limiting value (p_(o)).
 3. A method as claimed in claim 1,further comprising in-operation level monitoring with the compressorswitched on, measuring the refrigerant superheat (dTü) at the evaporatorof the system, and determining whether the measured superheat (dTü) liesabove a predetermined limiting value (dTü_(G)), as an indication ofimproper filling.
 4. A method as claimed in claim 1, wherein the systemcomprises an air-conditioning system employing CO₂ as refrigerant.
 5. Amethod as claimed in claim 4, wherein, the refrigerant superheat (dTü)at the evaporator is measured by using the difference between arefrigerant temperature (T_(KVA)) measured on the evaporator outlet sideand a refrigerant temperature (T_(KVE)) measured on the evaporator inletside, or by using the difference between a temperature (T_(LVA)),measured on the evaporator outlet side, of a medium led over theevaporator for the purpose of cooling the medium, and the refrigeranttemperature (T_(KVE)) measured on the evaporator inlet side.
 6. A methodas claimed in claim 4, wherein the air-conditioning system comprises anautomotive air-conditioning system.
 7. An apparatus for refrigerantlevel monitoring in a refrigerant circuit of an air-conditioning orheat-pump system having a compressor and a refrigerant which may,depending on the operating point, be operated in the supercriticalrange, the apparatus comprising: at least a system for standstill levelmonitoring with the compressor switched off, comprising detectors formeasuring both the pressure (p_(KM)) and the temperature (T_(KM)) of therefrigerant, and a calculation circuit for determining whether themeasured refrigerant pressure lies below a temperature-independentpredetermined minimum pressure value (p_(min)) or whether the measuredrefrigerant temperature lies above a predetermined maximum saturationtemperature value (T_(S)) and the measured refrigerant pressure liesoutside a predetermined target pressure range (p_(u), p_(o)), in eachcase as an indication of improper filling.
 8. An apparatus as claimed inclaim 7, further comprising a system for in-operation level monitoringwith the compressor switched on, comprising detectors for measuring therefrigerant superheat (dTü) at the evaporator of the system, and acalculation circuit for determining whether the measured superheat (dTü)lies above a predetermined limiting value (dTü_(G)), as an indication ofimproper filling.
 9. An apparatus as claimed in claim 7, wherein thesystem comprises an air-conditioning system employing CO₂ asrefrigerant.
 10. An apparatus as claimed in claim 9, wherein theair-conditioning system comprises an automotive air-conditioning system.11. An automotive vehicle, comprising an air-conditioner having arefrigerant circuit including a compressor and a refrigerant comprisingCO₂ which may, depending on the operating point, be operated in thesupercritical range, and a system for refrigerant level monitoring inthe refrigerant circuit comprising an apparatus as defined by claim 7.12. An automotive vehicle as claimed in claim 11, further comprising asystem for in-operation level monitoring with the compressor switchedon, comprising detectors for measuring the refrigerant superheat (dTü)at the evaporator of the system, and a calculation circuit fordetermining whether the measured superheat (dTü) lies above apredetermined limiting value (dTü_(G)), as an indication of improperfilling.